Production of ethanol by microorganisms provides an alternative energy source to fossil fuels and is therefore an important area of current research. It is desirable that microorganisms producing ethanol, as well as other useful products, be capable of using D-xylose and L-arabinose as carbon sources since these are the predominant pentose sugars in hydrolyzed lignocellulosic materials, which can provide an abundantly available source of carbon substrate for biocatalysts to use in fermentation.
Zymomonas mobilis and other bacterial ethanologens which do not naturally utilize D-xylose or L-arabinose may be genetically engineered for utilization of these sugars. To provide for D-xylose utilization, strains have been engineered to express genes encoding the following proteins: 1) D-xylose isomerase, which catalyzes the conversion of D-xylose to D-xylulose; 2) xylulokinase, which phosphorylates D-xylulose to form D-xylulose 5-phosphate; 3) transketolase; and 4) transaldolase (U.S. Pat. No. 5,514,583, U.S. Pat. No. 6,566,107; Zhang et al. (1995) Science 267:240-243). To provide for arabinose utilization, genes encoding the following proteins have been introduced: 1) L-arabinose isomerase to convert L-arabinose to L-ribulose, 2) L-ribulokinase to convert L-ribulose to L-ribulose 5-phosphate, and 3) L-ribulose-5-phosphate 4-epimerase to convert L-ribulose 5-phosphate to D-xylulose 5-phosphate (U.S. Pat. No. 5,843,760).
Following introduction of the D-xylose utilization pathway genes, utilization of D-xylose is typically not optimal. Zymomonas strains genetically engineered for D-xylose utilization have been adapted for growth on D-xylose containing medium to obtain strains with improved use of D-xylose (U.S. Pat. No. 7,223,575 and U.S. Pat. No. 7,741,119). Further genetic modifications of the Zymomonas genome which improve D-xylose utilization have been disclosed in US 2013-0157331 and US 2013-0157332.
There remains a need for Zymomonas strains that have effective utilization of D-xylose and/or L-arabinose, and in addition of both D-xylose and L-arabinose, to enhance ethanol production, particularly in a medium containing hydrolyzed lignocellulosic biomass.